1. Field of the Invention
This invention relates to an image recording apparatus, and particularly to sheet size detector for a sheet container which records a sheet by installing a plurality of sheet containers within a sheet feed section thereof.
2. Discussion of the Related Art
An image recording apparatus such as an electronic copying machine or a laser printer may record an image on a sheet selected from a plurality of sheet containers, where each container is for holding sheets of varying sizes. Different sheet sizes are selected depending upon data such as a sheet size, or a rate of reduction/enlargement. Thus, a number of sheet containers are provided as part of the image recording apparatus.
In ordinary electronic copying machines, sheet size data supplied from each sheet container serves as a basis for adjusting a reference operation interval of detectors disposed along a sheet transfer path as well as the operation timing of a document handler and a jam detection system which are linked with the reference operation interval.
Means for automatically identifying the size of an image to be formed on a photoreceptor based on the sheet size data is also employed.
The typical sheet container used with the image recording apparatus includes a sheet feed cassette and a sheet feed tray. The sheet container is often dedicated to regularsized sheets such as A-size and B-size sheets. The sheet feed cassette may be provided with means for displaying the sheet size data of the sheet contained in the cassette, as disclosed for example, in Japanese Patent Unexamined Publication No. 218333/ 1987.
When the sheet feed cassette is installed in the image recording apparatus, the sheet size data is read and stored in a controller of the image recording apparatus so that the data will be linked with the copying operation of the sheet. Such related art sheet feed cassettes are effective in accommodating copying needs when sheet sizes are not diverse, and when it is not necessary to prepare a large number of cassettes. However, in the above-described related art device, an image recording apparatus adapted for optionally using sheets of various sizes must employ a multiplicity of sheet containers, thereby complicating their management.
Thus, a universal tray, which is a sheet feed tray capable of containing any size sheet, is sometimes used in lieu of the sheet containers dedicated to the regular-size sheets. As disclosed, for example, in Japanese Patent Unexamined Publication No. 175330/1987, a universal tray is provided with sheet positioning members such as sheet guides to keep a sheet of a desired size at a sheet feed position.
With this related art tray, the size of the sheet contained in the universal tray is identified by the controller of the image recording apparatus based on data transmitted, for example, from sensors that have detected the sheet guide position. Therefore, the use of the universal tray allows sheets of various sizes to be copied with a comparatively small number of trays.
The universal tray may also be effectively used in connection with an image recording apparatus such as one that has a sheet container with a relatively small sheet capacity. With this arrangement, a large number of copies can automatically be produced with ease by causing sheets of the same size to be contained in a plurality of such sheet containers.
However, as described above, the number of sensors provided in the universal tray for detecting the sheet guide positions is also limited, and it often happens that, unlike regular-size sheets, irregular size sheets cannot be detected correctly.
To overcome the above circumstances, the use of means for detecting an infinite amount of sheet guide positions may be conceivable in view of Japanese Patent Unexamined Publication No. 185730/1988. In this related art example, the sheet guide positions are detected as resistance through a variable resistor means.
Thus, the above-described sheet guide position means or sheet size detecting means, sheets of any desired size can be contained in and fed from a universal tray and the needs of the image recording apparatus can be met easily and correctly.
However, the ordinary image recording apparatus may use not only regular-size sheets grouped by the millimeter such as A3, A4, B3, and B4, but it may also use other sheet sizes grouped by the inch such as a so-called letter-size sheet.
Typically used sheets and their sizes are as shown in Table 1.
TABLE 1 ______________________________________ Length (mm) Group Group Sheet size SE LE A B SA SB ______________________________________ 3" .times. 5" 76.2 127 x 1 Postcard 100 148 x 1 A6 105 148 x 1 B6 128 182 5.5" .times. 8.5" 139.7 215.9 2 1 SEF x 1 2 LEF A5 148 210 x 6" .times. 9" 152.4 228.6 B5 ISO 176 250 7.25" .times. 10.5" 184.2 266.7 2 1 SEF B5 182 257 x 1 2 LEF 8" .times. 10" 203.2 254 8" .times. 10.5" 203.2 266.7 8" .times. 13" 203.2 330.2 8.27" .times. 10.63" 210 270 2 SEF A4 210 297 x x 2 LEF 8.27" .times. 13" 210 330.2 8.46" .times. 19.7" 215 273 8.46" .times. 10.83" 215 275 8.46" .times. 11" 215 279.4 8.46" .times. 12.4" 215 315 8.46" .times. 13" 215 330.2 8.46" .times. 14" 215 355.6 8.5" .times. 11" 215.9 279.5 x 2 2 SEF (letter) 2 3 LEF 8.5" .times. 12.4" 215.9 315 8.5" .times. 13" 215.9 330.2 8.5" .times. 14" 215.9 355.6 x 3 2 (legal) Special B4 252 358 10" .times. 15" 254 381 B4 257 364 x 3 3 11" .times. 17" 279.4 431.8 x 4 3 A3 297 420 x 4 B3 364 515 A2 420 594 17" .times. 22" 431.8 558.8 Visiting card 55 91 ______________________________________
As shown in Table 1, the ordinary image recording apparatus uses sheets of a multiplicity of sizes. These sheet sizes can be classified roughly into two groups: a regular-size such as A-size and B-size specified by Japanese Industrial Standards (JIS), and an inch-size.
However, even if a plurality of universal trays are installed in the sheet feed section of the image recording apparatus and the detected sheet size data is applied from each tray to the controller, it often happens that the controller cannot identify the sheet size correctly. For example, sheets whose sizes are so close to each other such as an inch-size sheet of 5.5".times.8.5" and a regular-size sheet of A5 cannot be distinguished from each other by the controller, when transferred both lengthwise and widthwise.
With respect to sheet size grouping, some users prefer to use the regular-size sheets while others, such as trading firms, use the inch-size sheets. The typical image recording apparatus has groups of sheets of different sizes in separate containers. That is, sheets belonging to different sheet size groups are not supplied in mixture.
However, as described above, the related art universal tray cannot distinguish correctly one sheet size group from the other, and therefore, it may misidentify sheets.
As described above, the ordinary image recording apparatus controls both the operation timing of the automatic document handler and an interval of writing an image to the photoreceptor based on the sheet size data of the sheet contained in each sheet container. Any erroneous sheet size data may aggravate the risk of abnormally shifting the image writing position or issuing a jam signal even under normal sheet transfer operation.
However, some sheet size detecting means provided in the related art universal tray cannot detect the sheet size correctly, making the entire system more prone to error.